你自己写的已经很好了!基本上都很正确,文法上也没有多大错误,只是最后几句更简洁、流畅,我试改如下：
My favorite fruit is watermelon.Watermelons are mainly produced in the central and southern parts of Taiwan.There are yellow and red watermelons.I like the red ones more because they are usually sweeter.Watermelon sometimes tastes like smoothie for its juiciness.Eating watermelon can always cool me down during the summer time.Of course,I like the watermelon juice as well.